As the quality of color copiers has improved, it has become easier to generate copies of a document that are indistinguishable from the original document. In many instances, the unauthorized copying of document content can have serious implications. For example, there is a concern that color copiers could be used to reproduce security documents, such as checks, stock certificates, automobile title instruments, and other documents of value, for illegal purposes.
One approach for authenticating documents and reducing unauthorized copying involves the use of what is commonly referred to as a void pantograph. Conventional techniques for creating pantographs involve forming printed dots (or other elements) of two different sizes and frequencies, which are used to create regions of similar tone, corresponding to a textual warning and background, respectively, in an original (authentic) document. Tone refers to the visual appearance produced by halftone dots, bars, or marks which cover at least a portion of a printed area and which usually have a frequency that is measured in dots, lines, or marks per inch. To provide a constant tone, the smaller elements have a higher frequency than the larger elements. Because the tone of the textual warning and the tone of the background pattern are selected to be the substantially the same, these two regions have a similar visual impact on an observer of the original document, and the textual warning is not readily perceived.
Upon copying, however, the situation changes. Since the response of an image sensor employed in the scanner is different from the response of the human visual system, changes in the relative tone of the two different areas will appear. These changes are due to the different frequency response of the sensor (with respect to the human eye) and also due to other, normally non-linear, effects, such as a detection floor or threshold, where signals below a certain level are simply “lost.” In general, the high frequency components are more strongly affected and attenuated. The difference in response of the scanner expresses itself as a relative change in tone in the copy and thus the hitherto invisible textual warning becomes visible. For example, in the resulting copy, only the larger printed dots are apparent. These larger dots spell out the word “void,” or other pre-determined textual warning.
In current techniques, the pantograph is applied to the substrate to create a pre-printed carrier. An image to be protected is then applied to the pre-printed carrier. These methods have generally been successful in protecting documents, and are sometimes combined with other techniques, such as the use of camouflage patterns, and the like. However, they are static in nature and thus in general are limited to generally valid, partly nondescript words like “void” or “copy.” Void pantographs are thus a well-known technique used for fraud protection.
FIG. 1 illustrates a prior art pictorial diagram 10 depicting a document with a void pantograph. That is, an original document 8 is shown in FIG. 1 prior to photocopying of the document. A photocopied document 9 (i.e., after the original document 8 has been photocopied with a color copier) of the original document 8 is shown with the words “VOID” displayed. A voice pantograph includes a pantograph screen with the word “VOID” hidden in it, created by using special screens and background designs. When photocopied by a color printer, the word “VOID” appears on the copied document, such as the photocopied document 9. A document with a void pantograph is thus more difficult to duplicate than one with a standard pantograph.
FIGS. 2-3 illustrate a prior art pantograph technique. In FIG. 2, a technique is shown involving a variable data pantograph using font pairs and Type 3 fonts with bitmap images. In the example shown in FIG. 2, the character “T” is shown with respect to two fonts 12 and 14 (e.g., respectively, font 1 left and font 2 right). A pantograph “T” is composed of the front pair written in the same space as shown in the arrangement 16 in FIG. 3. The goal is for the original “T” to appear as one level in the original, but be readable on the copy. The smaller and bigger dots are used in a conventional pantograph technique.
Another conventional example of a void pantograph technique is disclosed in U.S. Pat. No. 7,869,090, which shows a variable data pantograph using font pairs. The advantage of this approach over a single font is that fewer total fonts are needed for calibration. Due to the binary representation of this approach, a new font pair is needed for each font size style and face. Thus, for example, for a single font, e.g., Xerox-Sans with 10 sizes (e.g., 18-72 points every 6 points), 4 styles (e.g., Roman, Italic, Bold, and Bold-Italic), one would need, for example, 10×4×2 pair×5 minimum calibration set=400 separate fonts. The minimum calibration is required due to typical printer variation (e.g., drift) where the dots vary from printer to printer and the same printer over time. This approach and the conventional techniques shown in FIGS. 2-3 are generally too cumbersome and impractical to implement in actual products.